Screw compressor

ABSTRACT

An improvement in the construction and operation of the capacity control slide gate for a screw compressor of the type having two helical intermeshing rotors in operative engagement with each other, radially surrounded by a cylindrical housing of 8-shaped cross-section, is disclosed. The novel slide gate is guided not only by the conventional trough-like recess in the compressor housing, but also by an opening in the outlet end closure of the compressor corresponding to the profile of the slide gate and by a piston rod attached to the slide gate and slidably journalled in the rotor housing.

United States Patent 1 Axelsson Sept. 9, 1975 [54] SCREW COMPRESSOR 3,756,753 9/l973 Persson et al A, 4l8/l59 x [75] Inventor: Rolf Axelsson, Norrkopmg, Sweden Primary Examiner wimam L. Freeh [73] Assignee: Stal Refrigeration AB, Norrkoping, Assistant Examiner-Gregory LaPointe Sweden Attorney, Agent, or FirmEric Y. Munson [22] Flled: June 28, 1973 ABSTRACT [2 1] Appl- 374,500 An improvement in the construction and operation of the capacity control slide gate for a screw compressor [30] Foreign Application Priority Data of the type having two helical intermeshing rotors in Jun6 30 9 Sweden 3645/72 operative engagement with each other, radially sur rounded by a cylindrical housing of 8-shaped cross- [52] us Cl 417/310; 418/201 section, is disclosed. The novel slide gate is guided not [51] hm z H F04]; 49/00. Folc 1/16 only by the conventional trough-like recess in the [58] Field of 418/159 6 417/283 compressor housing, but also by an opening in the outi 417/316 let end closure of the compressor corresponding to the profile of the slide gate and by a piston rod attached [56] References Cited to the slide gate and slidably journalled in the rotor UNITED STATES PATENTS housmg' 3,088,659 5/1963 Nilsson ct al. 4|s/159 7 4 Drawmg F'gures 3,3l4,597 4/l967 Schibbye A H 418/159 3,432,089 3/1969 Schibbyc. 418/201 X 3,738,780 6/l973 Edstrom 4l8/l59 X BACKGROUND OF THE INVENTION In known screw compressors of the type having two helical rotors in operative engagement with each other, radially surrounded by an 8-shaped cylindrical housing, the housing is provided with opposite end closures, one with an inlet port and the other an outlet port for the gaseous medium to be compressed. For continous capacity control of the compressor, a section of the cylindrical housing between the rotors is constructed as an axially movable slide gate, the outer surface of which is cylindrically convex and rests in a corresponding, trough-like recess in the housing, this recess extending at its bottom into an opening in the compressor housing which is exposed when the slide gate is displaced toward the outlet end of the compressor.

Thus, the slidegate comprises a body defined by two concave and one convex cylindrical surfaces, which, in prior art compressors, is joumalled primarily in a drilling in the rotor housing corresponding to the convex cylindrical surface of the slide gate. The slide gate therefore is required to extend more than 180 (usually about 200") of the total cylinder surface, and as a consequence, becomes rather large and clumsy. Also, rotation of the slide gate in the housing drilling must be prevented by the use of guide bosses or the like, and replacement of a slide gate in an already assembled known compressor is troublesome and difficult.

SUMMARY OF THE INVENTION The present invention has as its primary object the provision of a slide gate for a screw compressor which is considerably simplified, less bulky, and much more easily exchanged or replaced than those now in use.

In addition to the guidance of the conventional troughlike recess in the compressor housing, the novel slide gate is further guided by an opening in the outlet end closure of the compressor corresponding to the profile of the slide gate and by a piston rod attached to the slide gate and slidably journalled in the rotor housing. This guidance system makes it possible for the slide gate to be much slimmer and less bulky. For example, the extent of the convex cylindrical surface of the slide gate may be kept to about 60, as contrasted to the more than 180 of the prior art. Preferably the respective arcs should extend along a curvature of less than 90.

Furthermore, the slide gate of this invention can be more simply fitted and replaced in the compressor otherwise already assembled.

These and further features of this invention will be described fully and clearly with reference to the accompanying drawings which illustrate a preferred embodiment of the screw compressor and slide gate.

DRAWINGS FIG. 1 is a longitudinal section through a screw compressor constructed according to this invention, taken along line AA of FIG. 2;

FIG. 2 is a view of the compressor as seen from its outlet end; and

FIGS. 3a and 3b are, respectively, a top plan view and an end view of the slide gate itself.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows the compressor housing, consisting of cylindrical body 1 and the end closures 2 and 3. The two helical rotors which rotate within the compressor housing have been omitted for the sake of clarity, but their construction is known and described, for example, in US. Pat. No. 3,283,996.

End closure 2 forms the inlet side of the compressor and contains inlet opening 4; end closure 3 forms the outlet side, with outlet opening 5. A flange 6 is clamped to end closure 3 for sealing purposes and for journallin g of the rotors (not shown).

In the middle of cylindrical housing 1 but in its lower side, slide gate 8 rests slidably in a trough-like recess in the compressor housing 1. The shape of slide gate 8 is clearly illustrated in FIG. 2 where it can be seen readily that its profile is defined at the top of two concave cylindrical surfaces, while its lower side is a convex cylindrical surface conforming to the trough-like recess in housing. Slide gate 8 is attached by spring snap ring 10 to piston rod 9, the righthand end of which is slidably journalled in a drilling in housing 1. Further guidance of slide gate 8 is provided in outlet opening 5 of end closure 3, as shown in FIG. 2.

Outlet opening 5 has an upper portion lying within the periphery of the two rotors which is conventionally shaped in approximate agreement with the grooves in the rotors. Below this section the edge of opening 5 is defined by two machined bosses 11,12 which fit exactly the upper surfaces of slide gate 8, which is thus guided at its top by bosses 11,12, at its bottom by the bottom 13 of the trough-like recess, and at its righthand end by piston rod 9. Bosses 11,12 also prevent axial rotation of slide gate 8 in either direction.

As mentioned above, slide gate 8 is displaceable and slidable axially, and in FIG. 1 is shown in its lefthand open position, exposing opening 14 in the bottom 13 of the recess in housing 1. The position of slide gate 8 is controlled by the piston 15 in cylinder 16, the pressure from piston 15 being transmitted to piston rod 9 through connecting rod 17 and perforated pressure plate 18 mounted on rod 17.

OPERATION The slide gate 8 and thus the compressor unit function in the following way: in the position shown in FIG. 1, gas passes from the intake 4 through the channels formed by the rotors, after which most of the gas passes through opening 14, the holes in pressure plate 18, and the return channel 19 back to inlet 4. Only a small quantity of gas will be compressed in the active part of the rotor grooves, i.e., the quantity of gas opposite the part of slide gate 8 which is within housing 1. This means that the lefthand end of slide gate 8 is influenced by the outlet pressure while its righthand is influenced by the intake pressure. During operation, therefore, slide gate 8 is urged by a resultant pressure to the right which endeavors to close opening 14. This pressure is counteracted by pressure from piston 15.

To operate piston 15 cylinder 16 is connected to the outlet and inlet side of the compressor by conduits 20 and 21 respectively, (shown diagrammatically), each provided with a shut-off valve. As shown in FIG. 1, cylinder 16 is opcratively connected to outlet opening 5 (the valve in conduit 20 is open) so that pressure in cylinder 16 applied via piston 15, connecting rod 17, pressure plate 18 and piston rod 9 overcome the previously mentioned ambient pressure on slide gate 8. It is clear that in order to accomplish this, piston must have a minimum area relative to the cross-sectional area of slide gate 8. Piston 15 should have a cross-sectional area at least double that of slide gate 8.

The position shown in FIG. 1 corresponds to the lowest capacity of the compressor since opening 14 is completely unobstructed,

The capacity of the compressor is increased by shutting off conduit and opening conduit 21 so that cylinder 16 is connected to inlet port 4. The pressure on piston 15 thereby disappears and pressure from the outlet side forces slide gate 8 to the right so that opening 14 is throttled. Full compressor capacity is attained when slide gate 8 is in its rightmost position, when cylindrical housing 1 is completely enclosed and uninterrupted and compression occurs along the entire length of the rotors.

The valves in conduits 20 and 21 are preferably operated by a pressure regulator (not shown) controlled by the pressure from the outlet side of the compressor. Such regulator control is well known, however, and does not fall within the scope of the present invention.

It is clear that the piston rod 9 and the pressure plate 18 are always kept substantially firmly against each other and no permanent connection is therefore necessary between these two parts. This is a great advantage from the manufacturing point of view, since no accurate fitting in radial direction between 9 and 18 is re quired.

The eccentric positioning of 8, 9 and 15-18 in relation to each other also enables the complete cylinder unit 15-18 to be placed in the inlet closure 2, below inlet 4 and the bearings, not shown, for the rotors in inlet closure 2. This cylinder unit was previously placed completely outside the compressor housing, resulting in an extremely bulky construction.

As previously mentioned, the construction shown permits slide gate 8 to be replaced easily by loosening spring snap ring 10, which is of importance for the following reason: the shape of slide gate 8 is shown most clearly in FIGS. 3a and 3b and it can be seen that the lefthand end of slide gate 8 is cut or bevelled on the upper side. By altering the extent of this bevelling, different volume ratios can be achieved in the compressor. By providing the compressor with a slide gate having the correct bevelling, the desired volume ratio can be achieved: strong bevelling giving a large outflow opening from the slide gate and thus low volume ratio, i.e., moderate outlet pressure, while slight bevelling will give a large volume ratio and thus considerable outflow pressure.

Since the same compressor with different slide gates can be used for different volume ratios, it is a great advantage to be able to assemble the compressor all at once and fit the correct slide gate only when the compressor is to be delivered or fitted on site.

The construction and operation of a preferred cmbodiment of this invention have been described; the scope of the invention is defined by the appended claims.

I claim:

1. A capacity control valve for a screw compressor having two intersecting bores defining a working cham ber in which a pair of intermeshing helical screws rotate to compress and entrap a working fluid passed from an inlet wall to an outlet wall in said working chamber, said control valve comprising:

a. a slide gate comprising a portion of the wall of said working chamber and being axially displaceable to regulate an opening for venting working fluid from the bottom of the working chamber;

b. said slide gate in cross section defining a triangular profile the base of which describes a convex shallow arc with its center located on a line of symmetry between the two helical screws and the two sides thereof describing two shallow concave arcs conforming to the peripheral contour of the helical screws, each of said ares defining a curvature of less than to form a substantially flat triangular member;

. said slide gate being guided in a recess in said working chamber which recess has a profile corresponding to the profile of said slide gate; and

d. a piston rod connected to said slide gate and slidably journalled in said working chamber for further guiding the slide gate.

2. A capacity control valve according to claim 1, wherein the slide gate is replaceably attached, so that a slide gate of different characteristics may be substituted to alter selectively the volume ratio of the compressor.

3. A capacity control valve according to claim 1, wherein the movement of the slide gate is controlled by a valve actuating piston movable in a pressure fluid cylinder, said piston and said cylinder being located at the inlet end of the compressor.

4. A capacity control valve according to claim 3, wherein said valve actuating piston and said cylinder are positioned eccentrically with respect to said piston rod of the slide gate and located adjacent the inlet wall of the working chamber.

5. A capacity control valve according to claim 3, wherein the displacement of the slide gate is accomplished by operatively connecting said pressure fluid cylinder to the outlet end and the inlet end, respectively, of the working chamber.

6. A capacity control valve according to claim 3, wherein the free end of the slide gate projects into the outlet portion of the compressor and is actuated by the surrounding pressure, while the free end of said piston rod of the slide gate rests freely against said valve actuating piston.

7. A capacity control valve according to claim 6, wherein the cross-sectional area of said valve actuating piston in said cylinder is at least double the crosssectional area of the slide gate. 

1. A capacity control valve for a screw compressor having two intersecting bores defining a working chamber in which a pair of intermeshing helical screws rotate to compress and entrap a working fluid passed from an inlet wall to an outlet wall in said working chamber, said control valve comprising: a. a slide gate comprising a portion of the wall of said working chamber and being axially displaceable to regulate an opening for venting working fluid from the bottom of the working chamber; b. said slide gate in cross section defining a triangular profile the base of which describes a convex shallow arc with its center located on a line of symmetry between the two helical screws and the two sides thereof describing two shallow concave arcs conforming to the peripheral contour of the helical screws, each of said arcs defining a curvature of less than 90* to form a substantially flat triangular member; c. said slide gate being guided in a recess in said working chamber, which recess has a profile corresponding to the profile of said slide gate; and d. a piston rod connected to said slide gate and slidably journalled in said working chaMber for further guiding the slide gate.
 2. A capacity control valve according to claim 1, wherein the slide gate is replaceably attached, so that a slide gate of different characteristics may be substituted to alter selectively the volume ratio of the compressor.
 3. A capacity control valve according to claim 1, wherein the movement of the slide gate is controlled by a valve actuating piston movable in a pressure fluid cylinder, said piston and said cylinder being located at the inlet end of the compressor.
 4. A capacity control valve according to claim 3, wherein said valve actuating piston and said cylinder are positioned eccentrically with respect to said piston rod of the slide gate and located adjacent the inlet wall of the working chamber.
 5. A capacity control valve according to claim 3, wherein the displacement of the slide gate is accomplished by operatively connecting said pressure fluid cylinder to the outlet end and the inlet end, respectively, of the working chamber.
 6. A capacity control valve according to claim 3, wherein the free end of the slide gate projects into the outlet portion of the compressor and is actuated by the surrounding pressure, while the free end of said piston rod of the slide gate rests freely against said valve actuating piston.
 7. A capacity control valve according to claim 6, wherein the cross-sectional area of said valve actuating piston in said cylinder is at least double the cross-sectional area of the slide gate. 